INTRODUCTORY CHAPTER. 



25 



Mollusca, there are some which are injurious, 

 but to no very serious extent : snails and slugs 

 do much damage in our gardens by their vo- 

 racity ; and many of the marine species, which 

 make their residence in wood and stone beneath 

 the surface of the sea, occasionally do us much 

 damage, by boring into dikes, piers, and even 

 ships. 



| Having thus given an outline of the nature 

 of Mollusca, or, in other words, of shells and 

 their inhabitants, in order that the reader might 

 be able to form a general idea of them, it is 

 now time to enter into their classification and 

 arrangement into different orders and genera, 

 so that he may have the power of becoming 

 more particularly acquainted with them, and of 

 deciding upon the nature, habits, and peculiari- 

 ties of any shell, and the mollusc which has 

 resided in it, by a simple inspection of its form 

 and appearance. It is to be observed, that it is 

 rarely indeed that the animal is to be met with 

 in collections, the shell only being retained as 

 an object worthy of preservation, and, conse- 

 quently, he will have to be guided in his re- 

 searches by the latter alone. 



